Quoting wraith2209, reply 13 I wanted to let them build other ships, but couldn't figure out how to do that. I don't think Sins code will allow that but I may be mistaken.
all you need to do is create an ability that can spawn ships. Sounds hard, but it is readily made if you use the Spawn Missle Platform/Siege platform as a base
Yes, i've done that, Blair pointed me in the same direction you mention, the deployable combat turrets, which are frigates with the engines nerfed, but the ability is there, all you have to do is change meshes and hudicons, name and description texts, for whatever you want. I made Frigate-Capitals that way that you could launch using the abilitydeploycombatturret.entity. You can also launch the other frigates without using the combatturret file, just the normal frigate one, and change the abilitydeployCombatTurret to something like abilitydeployPsiScout and it will. Inside the ability you can also set a timer for the ship if you want it to expire, like say 180 seconds, or -1 if you want it to be permanent.

Its way cool, try it, or if you like, check out my mini-mod its the Songs of a Crystal Sun 3.4 on WinCustomize, and just take a look at the code, its already there, i just plugged in values and switched meshes and pics and reference things found in the string folder, where everything gets its name and discription for the game.

Hope i dont sound like im touting my own horn, im not that good, it was already there to do, and it made it interesting to see it in game 
-Teal